1. Field of the Invention
This invention relates to the polymerization of olefins and more particularly relates to catalyst compositions useful for polymerizing one or more monomers comprising ethylene to polymers having a narrow molecular weight distribution and a good balance of physical properties.
2. Description of the Prior Art
It is known that catalysts of the type variously described as coordination, Ziegler, Ziegler-type, or Ziegler-Natta catalysts are useful for the polymerization of olefins under moderate conditions of temperature and pressure. It is also known that the properties of the polymers obtainable by the use of such catalysts, as well as the relative economies of the processes used to prepare the polymers, vary with several factors, including the choice of the particular monomers, catalyst components, polymerization adjuvants, and other polymerization conditions employed.
During the years since Ziegler catalysts were first publicly disclosed, there has been a considerable amount of research conducted on the use of such catalysts; and numerous publications have resulted from that research. These publications have added much to the knowledge of how to make various types of olefin polymers by various types of processes. However, as is apparent from the amount of research on Ziegler catalysis that is still being conducted throughout the world, as well as the number of patents that are still being issued to inventors working in the field of Ziegler catalysis, the means of attaining certain results when polymerizing olefins with Ziegler catalysts are still frequently unpredictable. The fact that this situation exists is sometimes due to the need to obtain a previously-unattainable combination of results; occasionally due to difficulties in obtaining the same results in a commercial-scale apparatus as in a laboratory-scale reactor; and often due to a polymerization parameter's having an effect, or side-effect, in a given type of polymerization process that is different from effects achieved by its use in prior art processes of a different type.
One aspect of Ziegler catalysis in which the need for further research has been found to exist has been in the field of preparing ethylene polymers having a narrow molecular weight distribution and a good balance of physical properties. Such polymers have particular application in the production of articles that are formed by injection molding; typically having molecular weight distributions such that their normalized V.sub.30 /V.sub.300 melt viscosity ratios are in the range of about 1.5 to 2.3, with the ratios in the lower portion of this range being generally preferred but difficult to attain with known processes that might otherwise be commercially feasible; and--like other polymers intended for commercial use--are desirably prepared by a process which is as economical as possible as well as being capable of producing a polymer having the desired properties.
There are, of course, known processes for preparing injection molding resins by polymerizing ethylene with the aid of Ziegler catalysts. However, the known processes typically suffer one or more of the disadvantages of lack of economy, inability to produce polymers having a suitable balance of properties, and/or unreliability in producing such polymers--particularly in commercial-scale operations. What is still needed is a catalyst which (a) is suitable for use in a gas-phase polymerization process, (b) is capable of yielding polymers having a narrow molecular weight distribution and a good balance of physical properties, and (c) has sufficient activity to be economically attractive.
British Pat. No. 1,489,410 (Monsanto) teaches gas-phase polymerization processes which, because of their use of supported Ziegler catalysts having a vanadium component and other factors, are commercially attractive processes. However, as taught in the patent, the processes are designed to result in the formation of polymers having the broad molecular weight distributions suitable for blow molding resins rather than the narrower molecular weight distributions needed for injection molding resins; and the patent itself does not suggest how its processes might be modified to result in the formation of polymers having narrower molecular weight distributions. Attempts to make the processes of the patent suitable for the preparation of injection molding resins by combining its teachings with the teachings of publications that discuss means of narrowing molecular weight distribution have not been successful. For example, polymers having a sufficiently narrow molecular weight distribution have not been obtained when Monsanto's preferred vanadium halides have been replaced with the alkoxy group-containing vanadium compounds which are within the scope of their patent and which U.S. Pat. Nos. 3,457,244 (Fukuda et al.) and 3,655,583 (Yamamoto et al.) teach to result in the production of polymers having narrower molecular weight distributions when unsupported catalyst systems are employed.
Fukuda et al. also teach that ethylene copolymers or terpolymers having narrow molecular weight distributions can be obtained by the use of an unsupported catalyst composition prepared by (1) mixing an alcohol containing 1 to 12 carbon atoms with VOCl.sub.3 and then (2) mixing the mixture thus obtained with an alkylaluminum compound in the presence of the monomers to be interpolymerized, and there are other patents, e.g., Stamicarbon's British Pat. No. 1,175,593 and U.S. Pat. Nos. 3,535,269 (Tanaka et al.), 4,071,674 (Kashiwa et al.), and 4,256,865 (Hyde et al.) which teach the use of catalyst compositions prepared by adding an alcohol at some stage during the catalyst preparation. However, although some of these patents are concerned with the production of polymers having narrow molecular weight distributions, none of them teaches a catalyst composition which satisfies the aforementioned need for a catalyst suitable for use in a commercially-attractive gas-phase polymerization process that is capable of producing injection molding-grade polymers having a good balance of physical properties.